1. Field of the Invention
The present invention relates to a magnetic random access memory (MRAM) which is a nonvolatile memory using a magnetoresistive effect and a driving method thereof and, more particularly, to a write/read sequence thereof.
2. Description of the Related Art
An MRAM is a device which performs memory operation by storing units of binary information “1” and “0”, using a magnetoresistive effect. The MRAM has high potential such as nonvolatility, high integration, high durability, and high-speed operation. For this reason, the MRAM that can replace an existing device such as a conventional DRAM or EEPROM is expected as an ideal device.
A typical example of the MRAM uses an MTJ (Magnetic Tunnel Junction) element that includes a multilayered film formed of metal magnetic bodies and an insulating body and utilizes a magnetoresistive change by a spin polarization tunneling effect in the multilayered film. Several memory cells have been proposed as MRAMs, which typically include a structure having a transistor inserted in series into one MTJ element to ensure a cell selectivity (ISSCC 2000TA7.2, ISSCC 2000TA7.3) (FIG. 19) and a cross-point structure (FIG. 20).
An MTJ element has a structure formed by sandwiching an insulating film by two magnetic films having conductivity. Two states are created depending on whether the spin directions in the two magnetic films that sandwich the insulating film are parallel or anti-parallel. More specifically, when the two magnetic films have the same magnetization direction, the magnitude of a tunneling current that flows through the insulating film is larger than that of a current that flows when the two magnetic films have reverse magnetization directions. In other words, when the two magnetic films have reverse magnetization directions, the resistance value between the two magnetic films having conductivity can be made larger than that when the two magnetic films have the same magnetization direction. For this reason, it is preferable for reading a signal to make the difference in resistance value as large as possible. In reading information from a memory cell, a current that flows in the two magnetic films through the insulating film is detected, or the current value is converted into a voltage and then the voltage is detected.
In writing information in a memory cell, one of the two magnetic films normally has a fixed magnetization direction and therefore is not influenced by the external magnetic field. The magnetic film whose magnetization direction is fixed is called a pinned layer. The other magnetic film has the same magnetization direction as that of the pinned layer or a magnetization direction reverse to that of the pinned layer depending on the applied magnetic field. The magnetic film whose magnetization direction can change is called a free layer.
The magnetization direction of the free layer is changed by a magnetic field generated by a current that flows to a bit line and write word line, which pass through each memory cell. At this time, a current having a half of the current amount necessary for a change in magnetization direction is supplied to each of the bit line and word line. Accordingly, any unselected memory cell is prevented from being rewritten (U.S. Pat. No. 6,081,445).
When information is to be read out from an MRAM memory cell, a voltage as low as several hundred mV is applied to the memory cell, and a current that accordingly flows is detected. If the voltage applied to the memory cell is too high, no sufficient MR (MagnetoResistance) ratio can be obtained. Hence, the voltage to be applied cannot be raised more than necessity.
On the other hand, when information in a memory cell is to be rewritten, a relatively large current of several ten mA or several mA must be supplied to the bit line and write word line to generate a magnetic field necessary for the rewrite. For this reason, an IR drop (voltage drop) occurs in the chip due to this operation. The power supply line and ground line in the chip may receive disturbance for a predetermined time.
Hence, to prevent any error in reading after a write for a memory cell, operation must be started after the variation in power supply line and ground line converges to some extent. For this reason, if operation is executed in, e.g., a repetitive write/read sequence for a write, read, write, read, . . . , a standby time must be inserted before every read, resulting in a low efficiency.
In consideration of the above problem in the prior art, it is preferable for a magnetic random access memory and a driving method thereof to optimize a sequence including a write and read for a memory cell, thereby totally shortening the necessary operation time.